Cant mask keyed footage?

I am trying to key an explosion so that it is behind my character. Before doing this I removed the black background of my explosion by using Shift channels. Without shift channels applied I can mask the layer fine, using subtract or even inverted add mask, but when masking the footage with the shift channels effect, the masking does nothing.

Is there a way to render my clip as a pre-keyed explosion? or just mask it in the same composition? Thanks